The present invention relates to improvements in magnetoresistive (MR) devices and methods for fabricating the same. More particularly, the MR device and the method of fabricating it are of particular utility in the manufacture of MR read heads of the non-shunt bias type for use in computer mass storage devices. Even more particularly the present invention provides a barrier layer to substantially obviate the prior significant problems of film delamination and "rosette" formation during fabrication of MR devices.
The design of a shunt biased magnetic head for use in magnetic tape subsystems utilizing magnetoresistive read elements is generally described in Cannon et al., "Design and Performance of a Magnetic Head for a High-Density Tape Drive", IBM J. Res. Develop., Vol. 30, No. 3 May 1986, the disclosure of which is hereby specifically incorporated by reference. In the formation of MR heads of the non-shunt bias type, a first alumina (Al.sub.2 O.sub.3 -aluminum oxide) layer is generally sputter deposited on a ferrite substrate. A nickel iron (NiFe) film and a titanium (Ti) overcoat are then deposited on the alumina layer. Utilizing photolithographic techniques, appropriate device dimensions are photo-defined and the structure is then ion-milled to define the head tracks. Following this step, a gap layer comprising a second layer of alumina is deposited, photo-defined and wet etched. A titanium/gold (Au) layer is then deposited on the track area as a conductive layer and interconnecting aluminum wires are ultrasonically bonded to the gold layer.
The ferrite substrate typically used in this process generally contains 5,000-6,000 pores/mm.sup.2 with pore sizes which range from approximately 0.5 micrometers up to approximately 5.0 micrometers. However, the deposition of alumina, nickel-iron and titanium layers on this porous substrate does not cover all of the pores. In the gap wet-etching step, the aluminum layer is etched with an etchant such as phosphoric acid. It has been found, that some of the etchant remains trapped in the ferrite pores when the substrate is subsequently covered with the gold conductive layer. Reaction between the trapped etchant and the first alumina layer overlying the ferrite substrate generates local internal pressure which pushes the upper layer films to form bubbles. The trapped gas eventually escapes creating a tiny hole at the center. The bubbles which are formed may also thereafter collapse as a result of the escaping gas, thereby forming "rosettes". The formation of "rosettes" in the MR head track areas causes the track resistance to change and induces failure due to electromigration. Such "rosettes" are a serious problem which becomes even more acute as track density increases on such MR heads.
Moreover, in the ultrasonic wire bonding process previously described, the alumina layer undergoes chemical changes through the transfer of ultrasonic energy. These changes promote preferential migration of iron (Fe) from the nickel-iron film toward the nickel-iron/first alumina interface. This preferential migration induces the formation of oxidic iron (FeO) at this interface. Such complex chemical mechanisms weaken the adhesion between the nickel-iron film and the first layer of alumina, which results in film delamination at this nickel-iron/alumina interface during the wire bonding step.
Both the formation of "rosettes" and the undesired ultrasonic induced film delamination affect the performance and yield of MR devices manufactured in accordance with the foregoing process. It is, therefore, highly desirable to eliminate or reduce the "rosette" formation and the ultrasonic induced film delamination at the nickel-iron/alumina interface.
Certain problems attendant to the rupture and the delamination of certain thin films in the processing and manufacture of MR devices have been recognized. U.S. Pat. No. 4,914,538 entitled "Magnetoresistive Read Transducer", issued Apr. 3, 1990 proposes the use of a thin film underlayer in conjunction with a thin film overlayer formed of material taken from the group consisting of titanium, chromium (Cr), tantalum (Ta), zirconium (Zr), hafnium (Hf) and titanium tungsten (TiW) to reduce etchant penetration and resultant delamination of tungsten (W) films. The thin film underlayer or overlayer is described as having a thickness within the range of 25-200 angstroms to prevent rupture and delamination of the relatively porous tungsten films utilized in the device described. U.S. Pat. No. 4,931,892 for "Long Life Magnetoresistive Head of the Non-Shunt Bias Type", issued on Jun. 5, 1990 advocates the use of a "sacrificial" material, such as titanium, in electrical contact with the MR element to extend the useful life of the more "noble" NiFe permalloy portions of the structure. Suggested thicknesses for the "sacrificial" material are on the order of less than 200 angstroms and materials such as titanium, tin (Sn), aluminum (Al), zirconium and chromium are described. Neither of the techniques described in the foregoing patents has provided or suggested a satisfactory solution to the undesirable formation of "rosettes" or delamination at the nickel-iron/alumina interface in the processing and manufacture of an MR element.
It is with respect to these and other considerations that the present invention has evolved.